The large scale production of ethanol has become increasingly important because of ethanol's use as a liquid fuel in gasoline additives and because it can be derived from renewable resources. Ethanol can be obtained from the anaerobic fermentation of simple sugars, however, the metabolic pathways by which yeasts are able to break down the sugar molecules differ as to the type of sugar fermented. Thus many strains of yeast which are able to ferment glucose and other six carbon sugars (hexoses) are unable to ferment xylose and other five carbon sugars (pentoses) because the metabolic pathway by which the pentose sugars are utilized is ineffective. Most yeasts are capable of fermenting glucose to ethanol in high yields, however, only a few yeast strains or yeast mutants have been identified as being able to directly ferment xylose. Thus, the production of ethanol by fermentation of simple sugars has been largely obtained from glucose and not xylose.
The source of the simple hexose sugars for fermentation into ethanol is typically cane sugar and starch containing grains. The use of cane sugar for fermentation into ethanol is economical only in countries where the climate is conducive to year round production and where there are low labor costs. Corn is predominantly used for this process in the United States, largely because of its availability and low price and the availability of tax credits for grain used for the production of ethanol. One alternate source that is starting to receive increased attention is lignocellulosic material, or alternatively referred to as biomass. Lignocellulose has great economic potential as a feed stock because large quantities of it can be readily obtained from agricultural residues, forest industry by-products, or paper industry waste. A disadvantage of lignocellulose as a feed stock for fermentation is that one of its three major components, hemicellulose, which comprises 20 to 30% of the lignocellulose, is predominantly composed of the biopolymer xylan whose monomer unit is xylose. Cellulose, the largest fraction of lignocellulose at 30 to 40% of the total material, is a biopolymer whose monomer unit is glucose. Thus, in order for lignocellulose to be an economical feed stock for ethanol production, both the xylose and glucose fractions must be fermented.
Although xylose is not directly fermentable by most yeasts, it is well known that its ketose isomer, xylulose, can be fermented by the same yeasts that are able to ferment glucose. The isomerization of xylose to xylulose can be accomplished with the catalyst xylose isomerase (also referred to as glucose isomerase or aldose isomerase). However, the proportion of xylose to xylulose in the reaction mixture when the enzymic isomerization reaches equilibrium is only about 5:1. High conversion rates of xylose can, therefore, be realized only by preventing the reaction from reaching equilibrium by continuously fermenting xylulose to ethanol as soon as it is formed. This latter goal can be achieved by conducting the isomerization and the fermentation steps concurrently in a single reactor.
The activity of the xylose isomerase is a very strong function of pH, with its optimum pH to be reported in the range of 7.0 to 8.0. However, unlike the isomerization reaction, the optimum pH of the fermentation reaction is between 4.0 and 5.0. Thus in order to optimize both the isomerization and fermentation process, it has been necessary to carry out these reactions in separate environments and at significantly different pH conditions. As an example, Gong et al., U.S. Pat. No. 4,490,468 discloses separate reactors for the isomerization and fermentation processes. This reference does disclose that these two processes could be performed simultaneously, but it does not propose any means or methods for maintaining the different optimum pH conditions for each reaction in a single reactor. In fact it proposes a compromised pH range of 6.8 to 8.0, preferably at 7.0 for both processes.